Flexure endurant composite elastomer compositions

ABSTRACT

A flexure endurant composition of an elastomer reinforced with a continuous phase of microporous, expanded polytetrafluoroethylene (ePTFE) having a ratio of elastomer to PTFE of approximately 1:1 to 50:1 on a volume basis. The composite is prepared by a coating process and the final articles are prepared by molding or vulcanizing. Such materials can be used to fabricate pump tubing, pump diaphragms, gaskets, bellows and other mechanical devices.

RELATED APPLICATIONS

The present application is a regular application based on U.S.Provisional Patent Application No. 60/074,703 filed Feb. 13, 1998.

FIELD OF THE INVENTION

A flexure endurant composition of elastomer reinforced with a continuousphase of microporous, expanded polytetrafluoroethylene (ePTFE).

BACKGROUND OF THE INVENTION

Silicone elastomers can be fabricated into many forms for use,illustratively, in the medical, electrical, and chemical industries.Articles such as peristaltic pump tubes, pump diaphragms, bellows, babybottle nipples, wire and cable sheaths, gaskets, and O-rings, forexample, are commonly made from silicone elastomers. Many of thesearticles, moreover, are used in applications that require repeatedflexing. For example, peristaltic pumps are used to transport liquidsand pastes through an elastomeric tube in which the tube is squeezedbetween a set of rotating rollers and a fixed pump housing. Siliconeelastomers are frequently used for peristaltic pump tubing. Uponrepeated flexure, however, the silicone rubber tubing develops cracks inthe side wall and ruptures catastrophically. The problem is exacerbatedwhen pumping fluids at elevated pressures and temperatures, leading toeven shorter pump tubing life. Clearly, a more durable substance isneeded for these purposes.

Silicones are a class of inherently flexible polymers withorganosilicon-oxygen repeating units which undergo bond rotation withlittle resistance. As a result, silicones possess excellent lowtemperature properties; however, their weak intermolecular andintramolecular polymer interactions result in poor tear strength andtoughness. As a result, silicone elastomers are often reinforced witheither particulate inorganic fillers or soluble silicone resin fillers.Inorganic fillers, such as fumed silica, for example, are known toincrease the tensile strength of dimethyl silicones by a factor of ten.Even the best silicone elastomers, however, are still limited toapproximately 1,300 psi tensile strength (ASTM D-412) and 250 ppi tearstrength (ASTM D-624 die B). Natural rubber, on the other hand, hassignificantly higher tensile and tear properties; however, it lacks manyof the useful silicone elastomer attributes of low temperatureflexibility, low dielectric loss, ozone resistance, low extractables,and radiation resistance. Thus, the need for an improved class ofreinforced silicone elastomers that combine the strength and toughnessof natural rubber with the useful attributes of silicone rubbercontinues to be unsatisfied.

Polytetrafluoroethylene (PTFE) is a polymer with excellent chemicalinertness coupled with high strength. In U.S. Pat. Nos. 3,953,566;3,962,153; 4,096,227; and 4,187,390, Gore teaches the lubrication ofPTFE powder and subsequent expansion of the PTFE into a microstructurecharacterized by nodes interconnected by fibrils. In these patents, Goreteaches the use of non-reactive fluids such as kerosene, naphtha, ormineral spirits as the lubricating fluid to aid in the extrusion of PTFEfine powder. The PTFE is extruded into a tape and dried to remove thenon-reactive lubricant. Finally, the extrudate is expanded to produce amaterial that has both high porosity and high strength.

Expanded PTFE (ePTFE) has also been prepared using reactive lubricants,as seen in Mitchell (U.S. Pat. No. 4,764,560), and Tu (EP 256,748; U.S.Pat. No. 5,071,609). Reactive lubricants consist of uncured silicone andoptionally a solvent such as kerosene, naphtha, or mineral spirits. ThePTFE fine powder is lubricated, extruded, and expanded. During theexpansion process, the silicone cures in situ to form aninterpenetrating polymer network (IPN) of PTFE and silicone elastomer.Such expanded structures have residual porosity, high strength, andmoderate resilience. Mitchell (U.S. Pat. No. 4,764,560; U.S. Pat. No.4,891,407; WO 87/02996) and Dillon (U.S. Pat. No. 4,832,009; WO9117205), for example, teach the use of heat curable dimethylsilicone toproduce a porous microstructure of interpenetrating matrices in curedform with moisture vapor transmission properties for use as bandages forsevere burn victims. The amount of curable silicone suggested inMitchell's '560 and '407 patents can range from as little as 1 part byweight per 100 parts of PTFE to as much 150 parts of silicone per 100parts of PTFE. Using Mitchell, however, it is not feasible to expandpaste extruded tape having more than 20 weight percent silicone into amicrostructure of interpenetrating matrices in cured form due to thelack of interconnection between nodes and fibrils which results in poorextrudate green strength. Thus, the compositions described by Mitchellpossessed little elasticity due to their relatively high volume fractionof PTFE when compared to the present invention.

Tu (U.S. Pat. No. 4,816,339) also describes the use of reactive andunreactive lubricants for the preparation of radially asymmetricvascular grafts having an elastomer content ranging from 5 to 120 weightpercent ratio of elastomer relative to PTFE. Tu teaches the use offluoroelastomers, silicone elastomers, and others. A typical processused for producing a multi-layer PTFE/elastomer implant includedblending the PTFE fine powder with the solvated elastomer, preforming amultilayered billet, extruding out of a die, curing the elastomer,expanding the composite, and forming an optional elastomeric polymercoating layer via a dip or spray coating operation. Other tubularprostheses have been developed by Mano (U.S. Pat. No. 4,304,010) whichcomprise a porous tubing of PTFE having a microstructure composed offibrils and nodes connected to one another by the fibrils, the fibrilsbeing radially distributed, and a porous coating of an elastomer boundto the outside surface of said PTFE tubing. The prosthesis can be vacuumimpregnated with elastomer solution to provide a coating thickness ofbetween 20 and 500 microns. The prosthesis has improved suture tearresistance when compared to previous art.

Tomoda (U.S. Pat. No. 4,133,927) teaches the lamination of ePTFE to anelastomer substrate wherein the porous sheet of ePTFE forms a layerhaving a thickness of about 0.05 mm or more on the surface of theelastomer substrate. The composite is formed by superimposing the porousfilm or sheet on a vulcanizable rubber elastomer substrate andsubjecting the material to heat and pressure sufficient to affectvulcanization of the rubber and adhesion between the porous PTFE and theelastomer substrate. In the case of fluorine-containing rubber, theresulting composite exhibits excellent chemical resistance. Tomoda doesnot teach the use of multiple layers of ePTFE to form a composite thatis capable of transferring stress on a molecular level throughout thebulk.

For many years, silicone elastomers have been modified with PTFE powderto increase their lubricity, thermal stability, and tear strength.Safford (U.S. Pat. No. 2,710,290) teaches the use of a minor portion ofsolid PTFE dispersed throughout the silicone to form randomlydistributed fibrils. He shows that the PTFE particles were elongated insitu within the silicone matrix by means of shear deformation action. Asa result, the tear strength, as measured by ASTM D-624 (die B), wasincreased from 65 ppi to 230 ppi. Konkle teaches in U.S. Pat. No.2,927,908 that PTFE can be used to increase tensile and tear strength inheat curable fluorinated organopolysiloxane elastomers. These compositeswere also characterized as fuel and oil resistant. These examples ofPTFE particles dispersed into silicone rubber are limited to less than25 weight percent due to the difficulty in processing of the rubber anddeterioration of the physical properties of the vulcanizate. Unlike thepresent invention, PTFE powder filled elastomers lack the continuouslayer of ePTFE whose microstructure can be characterized by nodesinterconnected by fibrils, and thus have inferior flexure resistance.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a compositeelastomer with superior flexure endurance wherein the ratio of elastomerto ePTFE ranges from 1:1 to 50:1 on a volume basis.

Another object of the present invention is to provide a chemicalresistant fluoroelastomer for applications that require flexure in thepresence of aggressive chemicals.

Still another object of the present invention is to provide a reinforcedelastomer useful for pump components, diaphragms, gaskets, seals,o-rings, belts, tubes, and bellows.

The present invention relates to flex endurant elastomer compositionsbased on elastomers reinforced with a continuous phase of microporous orexpanded polytetrafluoroethylene (ePTFE). More particularly, theinvention relates to a mixture of ingredients comprising (1) a liquidelastomer convertible to a cured, solid elastic state and (2) a minorportion of ePTFE having a continuous microstructure characterized bynodes interconnected by fibrils.

There are also provided methods for fabricating these flex endurantcomposites of an elastomer and ePTFE. The processes involve coatingePTFE material with liquid elastomer, wrapping the impregnated materialaround a mandrel, and, optionally, applying heat and/or pressure tovulcanize the elastomer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Scanning electron micrograph (SEM) of ePTFE at 5,000 timesmagnification showing the continuous microstructure characterized bynodes interconnected by fibrils.

FIGS. 2A, 2B and 2C are schematic representations of gravure coatingprocesses for impregnating ePTFE with liquid elastomer.

FIG. 3. Flexure endurance of silicone/ePTFE composite elastomer versussilicone rubber and ePTFE GORE-TEX® GR® sheeting using ASTM D2176-69.

FIG. 4. SEM of elastomer composite having a silicone content of 90volume %.

FIG. 5. Pressure handling capability of silicone/ePTFE compositeelastomer compared to silicone rubber and thermoplastic elastomer, soldunder the trademark MARPRENE®.

FIG. 6. Flow rate as a function of time for silicone/ePTFE compositeelastomer pump tubing versus extruded silicone heat cured rubber pumptubing.

FIG. 7. Schematic representation of blow molding tool used to fabricateelastomer composite bellows.

FIG. 8. Illustration of multicavity mold with composite elastomerrelease film used to replicate molded parts.

DESCRIPTION OF THE INVENTION

The compositions of the present invention provide superior flexureendurant elastomer composites. The use of expanded PTFE provides amicrostructure of nodes interconnected by fibrils that serve todistribute stress from one part of the elastomer to another, on amolecular level. This composite is formed in the following manner:

First, an expanded PTFE (ePTFE) material is produced, such as throughthe methods described in U.S. Pat. No. 3,953,566 to Gore. For instance,the ePTFE membrane may be formed from a mixture of PTFE resin (having acrystallinity of about 95% or above) and a liquid lubricant (e.g. asolvent of naphtha, white oil, mineral spirits, or the like). Themixture is thoroughly blended and then formed into a pellet. The pelletis extruded into a tape using a ram-type extruder. Subsequently, thelubricant may then be removed through evaporation in an oven. Theresulting tape may then be subjected to either uniaxial or biaxialstretching at a temperature of less than 327° C. to impart the desiredamount of porosity and other properties to the membrane. Stretching maybe performed through one or more steps. The resulting membrane may thenbe subjected to a sintering temperature above 345° C. (i.e. the meltingtemperature of PTFE) to amorphously lock the membrane in its expandedorientation. The result is a porous structure, as show by the scanningelectron micrograph (SEM) in FIG. 1, which depicts the polymeric nodes,interconnected by fibrils. Typical properties of a structure comprise anaverage fibril length between nodes of 0.05 to 30 microns (preferablybetween 0.2 and 30 microns), and a void volume of 20 to 90%. As shouldbe evident from the following description, the precise properties anddimensions of ePTFE structures employed with the present invention are afunction of the application. Particulate fillers can also beincorporated into the ePTFE structure as taught by Gore (U.S. Pat. No.4,096,227; U.S. Pat. No. 4,187,390). Fillers, such as fumed silica,provide an active site for either covalent linking of the elastomer tothe ePTFE or for hydrogen bonding of the elastomer to the filler. Thegeneral membrane properties suitable for use with the present inventionshould include medium to high porosity, and wettability by varioussolvents, such as methylene chloride, toluene, and/or acetone.

Substrate material made through one of the above described methods andsuitable for use in the present invention is commercially available in awide variety of forms from a number of sources, including under thetrademark GORE-TEX® from W.L. Gore & Associates, Inc., Newark, Del.

The elastomers used in the present invention may be natural or syntheticin origin. Examples of common synthetic elastomers include silicones,urethanes, nitrile rubber, styrene-butadiene-styrene (SBR), chloroprene,phosphazenes, fluoroelastomers, perfluoroelastomers, perfluoropolyetherelastomers, having a rubbery elastic modulus of less than 10⁷ Pa. In apreferred embodiment of this invention, solventless liquid elastomerscan be used that simplify processing and are more environmentallyacceptable. Such liquid elastomers are commercially available in a widevariety of forms from a number of sources, including under the trademarkSILASTIC® from Dow Corning Corporation, Midland, Mich. and as a seriesof room temperature vulcanizates (RTV) from General Electric Silicones,Waterford, N.Y. A series of liquid perfluoro polyether elastomers areavailable under the trademark SIFEL® from Shin Etsu Chemical, Tokyo,Japan.

The organosilicone elastomer component can be cured using variousmechanisms; however, hydrosilylation reactions between vinyl and hydridefunctional polymers is the preferred approach and will be referred to asan addition cure system in this application. These elastomers arepreferably formulated to be solventless, liquid materials in the uncuredstate at room temperature. Addition cure elastomers typically consist oflinear polymer, reinforcing agent, crosslinker, catalyst, inhibitor,and, optionally, an adhesion promoter.

Linear silicone polymers used in this invention have viscosities up to1,000,000 cps or more at 25° C. and are, preferably, functionalized withdimethylvinyl groups at the ends of the polymer chains and/orvinylmethyl siloxane repeat units in the polymer backbone. Solventlesscoating techniques can be employed if the polymer viscosity ismaintained between 10 and 100,000 cps at 25° C. and, especially, whenthe viscosity is between 1,000 and 50,000 cps at 25° C.

Reinforcing and/or extending agents include insoluble particulatefillers such as fumed silica, precipitated silica, ground quartz andcarbon black. These fillers are preferably treated with silane couplingagents to render them hydrophobic and thus more compatible with thesilicone base polymer. Soluble reinforcing agents includepolyorganosiloxane resins of the type disclosed by Modic (U.S. Pat. No.3,436,366) and Nelson (U.S. Pat. No. 3,284,406). Silicone resins arehyperbranched copolymers of one or more of M (trimethyl siloxy), D(dimethyl siloxy), or T (methyl siloxy) units condensed with Q(silicate) units. Also, one or more of the M, D, or T units could befunctionalized with vinyl, hydride, trifluoropropyl, phenyl, or otheralkyl groups. Preferred compositions are MDQ resins with vinylfunctionality on either the M or the D siloxane units. The use of thesesoluble resins often permits the formulation of solventless materialsthat are reinforced but are not thixotropic as in the fumed silicareinforced materials. Polyorganosiloxane resin materials admixed withlinear polymers are preferably formulated to have viscosities below100,000 cps for solventless application. High viscosity materials can beemployed with the use of solvents.

Crosslinkers such as organohydrogen polysiloxanes can be used in theinstant invention in either linear or resinous form. Linear crosslinkersare preferably functionalized with dimethylhydrogen groups at the endsof the polymer chains and/or methylhydrogen siloxane repeat units in thepolymer backbone. The polyorganosiloxane resin crosslinkers arehyperbranched copolymers of one or more of M, D, or T units condensedwith Q units. One or more of the M, D, or T units could befunctionalized with vinyl, hydride, trifluoropropyl, phenyl, or otheralkyl group. Both the linear and resinous crosslinkers have viscositiesbetween 25 and 10,000 cps at 25° C., with a preferred range between 50and 1,000 cps at 25° C.

Inhibitors used to control the cure rate of addition cure RTVs at eitherroom temperature or elevated temperatures includepolymethylvinylcyclosiloxane having three to six methylvinylsiloxaneunits per molecule. Another class of preferred inhibitors are theacetylenic compounds (U.S. Pat. No. 3,445,420), particularly2-methyl-3-butyn-2-ol.

Catalysts for the addition cure elastomers include precious metals suchas platinum, rhodium, palladium, and others. These precious metals canbe solubilized or complexed in solution as taught by Karstedt in U.S.Pat. No. 3,814,730, by Ashby in U.S. Pat. No. 3,159,601, and Lamoreauxin U.S. Pat. No. 3,220,970. A preferred catalyst for addition cureelastomers is platinum solubilized in vinyl functional silicone polymerat a level of between 0.5 and 50 wppm platinum in the final elastomer.Organic peroxides can be used to crosslink vinyl containing elastomers.Preferred peroxides include benzoyl peroxides, dicumyl peroxide,di-t-butyl peroxide, and others familiar to one skilled in the art.

Condensation cure elastomers can also be employed in the describedinvention. These elastomers are formed from the condensation ofhydrolyzable silicone polymers such as hydroxyl functional dimethylsiloxane. Catalysts used to crosslink these elastomers include tin andtitanium based compounds. Preferable condensation catalysts includedibutyl tin dilaurate and dibutyl tin oxide.

Polyorganosilicone materials can be formulated to be either one ormulticomponent in nature. A preferred two component system includes an“A” and a “B” side. The “A” preferably includes the linear polymer,reinforcing agent, and catalyst. The “B” side preferably includes thelinear polymer, reinforcing agent, inhibitor, crosslinker, andoptionally a silane coupling agent. These systems are commerciallyavailable in various ratios of “A” to “B” including, but not limited to10:1, 9:1, and 1:1. In some cases, a one part material can be formulatedto include the linear polymer, reinforcing agent, crosslinker,inhibitor, catalyst, and, optionally, a silane coupling agent with theselection of an appropriate inhibitor.

The perfluoro polyether based SIFEL® elastomers involve thehydrosilylation between vinyl and hydride functional polymers withperfluoro polyether repeating units for the backbone as taught in U.S.Pat. Nos. 5,288,829; 5,554,779; 5,314,981; and 5,292,848. Theseelastomers are preferably formulated to be solventless, liquid materialsin the uncured state at room temperature. These perfluoro polyetherbased RTVs consist of linear polymer, crosslinker, catalyst, inhibitor,and, optionally, reinforcing materials and adhesion promoter(s).

Once suitable ePTFE and elastomer precursor materials are obtained, thefollowing processing can be performed to produce the composite materialof the present invention. The ePTFE membrane can be coated by any one ofa variety of methods including gravure coating to impregnate the porousstructure with elastomer, as shown schematically in FIGS. 2A, 2B and 2C.The ePTFE membrane is paid out onto a gravure roll (A), whereupon it iswetted with liquid elastomer. The liquid is driven into the porousstructure with the application of pressure by means of rubber roll (B)pressing against the gravure roll (A). Optionally, the impregnatedmembrane can be further conveyed, as shown in FIG. 2B, to either achrome roll (C) to chrome roll (D) gap for the application of a top coatof liquid elastomer, or as shown in FIG. 2C, to a rubber roll (B) tochrome roll (C) nip. The coating thickness can be varied to producecomposites of desired elastomer content. The coated structure is theneither taken up in the uncured state around a cylindrical mandrel (E) toa desired wall thickness or passed through a convection oven to cure theePTFE reinforced elastomer membrane. In the uncured state, the elastomerimpregnated ePTFE can either be vulcanized around a mandrel to form atubular article, such as pump tubing, or removed from the mandrel bycutting along the longitudinal axis to produce a flat sheet of uncuredmaterial. The uncured material can then be die cut to produce preformsfor compression molding into articles of complex shape such as adiaphragm, O-ring, gasket, etc. Another approach is to take up thecoated membrane onto a mandrel and slice the material into tapes ofdesired width. The tapes can then be wrapped around a mandrel usingfilament winding techniques to generate three dimensional objects ofirregular shape and unlimited length.

In The ratio of elastomer to expanded PTFE should be sufficient torender the article elastomeric without adversely affecting flexureendurance. Compositions too lean in elastomer provide articles thatbehave in a plastic-like manner. They often exhibit considerable creep,hysteresis, and lack of resilience or rebound. Compositions that are toorich in elastomer do not benefit from the microstructure of nodesinterconnected with fibrils that provide the unique flex life andstrength of the instant invention. Thus, the composition most suitablefor maximum fatigue life ranges from approximately 1:1 elastomer to PTFEto approximately 50:1 elastomer to PTFE by volume. More preferably theratio ranges from 4:1 to 25:1 elastomer to PTFE by volume.

Elastomeric articles can be fabricated from these composites by anynumber of molding techniques including compression molding, blowmolding, extruding, and laminating. The preferred addition cureelastomers can be heat cured to accelerate fabrication. Articles such aspump diaphragms, O-rings, gaskets, dosing valves, tubes and other shapedarticles can be readily formed by molding multilayered preforms.

The present invention comprises a composite in which the composite has aplurality of expanded PTFE layers. The PTFE layers are impregnated withat least one elastomer and the impregnated PTFE layers are adheredtogether by layers of elastomer. The ratio of a thickness of anelastomer layer to a thickness of an impregnated expanded PTFE layer is6.5:1 or less. The at least one elastomer and the layers of elastomereach independently comprise a natural or synthetic elastomer and can beat least one of a methyl silicone, a phenyl silicone, a fluorosilicone,a fluoroelastomer, a perfluoroelastomer, a perluoro polyether elastomer,or combinations thereof.

In the present composite articles, the volume ratio of elastomer toexpanded PTFE is at least 80%. The expanded PTFE can include at leastone filler, wherein the at least one filler can be, for example, fumedsilica, precipitated silica, colloidal silica or carbon black.

In some possible embodiments the expanded PTFE layers can be plasmatreated or the expanded PTFE layers can contain a silane coupling agent.

The present composite materials can be used to make gaskets, o-rings,bellows and pump diaphragms.

The present invention further comprises a composite tube in which the istube comprises a plurality of expanded PTFE layers. The expanded PTFElayers are impregnated with at least one elastomer and the impregnatedlayers are adhered together by layers of elastomer. The ratio of thethickness of an elastomer layer to the thickness of an impregnatedexpanded PTFE layer is 6.5:1 or less. The tube can have a diameter ofelongation of less than 35% of the original diameter when subjected to125 psi internal pressure at 25° C.

The at least one elastomer and the layers of elastomer eachindependently can comprise a natural or synthetic elastomer or can be anorganosilicone such as, for example, a methyl silicone, a phenylsilicone, a fluorosilicone, a fluoroelastomer, a perluoroelastomer, aperfluoro polyether elastomer, or combinations thereof. The volume ratioof elastomer to expanded PTFE is at least 80%. The expanded PTFE used tomake the tubes may include at least one filler such as, for example,fumed silica, colloidal silica, carbon black, or combinations thereof.The expanded PTFE layers of the tube can include a plasma treatment andcan also have a silane coupling agent treatment thereon. The tube madein accordance with this invention can be operated in a peristaltic pumpfor at least 100 hours at 200 rpm at a pressure of at least 30 psi.

In a specific embodiment of the present tube, the tube comprises atleast 31 layers of expanded PTFE in which the layers are impregnatedwith at least one organosilicone elastomer. The impregnated layers areadhered together by layers of organosilicone elastomer in which theratio of the thickness of an elastomer layer to the thickness of animpregnated expanded PTFE layer is 6.5:1 or less.

The present tubes are non-contaminating, steam sterilizable and can beoperated in a peristaltic pump for at least 100 hours at 200 rpm at apressure of at least 30 psi.

The present composite articles can comprise a plurality of expanded PTFEmembrane layers in which the layers have a thickness of between about0.2 mil and about 3.5 mil and the layers are impregnated with at leastone elastomer. The impregnated layers are adhered together by layers ofelastomer and the volume ratio of elastomer to expanded PTFE in thearticle is at least 3:1. The at least one elastomer and the layers ofelastomer each independently can comprise a natural or syntheticelastomer or can be an organosilicone such as, for example, a methylsilicone or a phenyl silicone, a fluorosilicone, a fluoroelastomer, aperfluoroelastomer, a perfluoro polyether elastomer, or combinationsthereof. The expanded PTFE of the composite article can include at leastone filler, such as, for example, fumed silica, colloidal silica, carbonblack, or combinations thereof, and the expanded PTFE layers can have aplasma treatment thereon and/or the expanded PTFE layers can include asilane coupling agent.

Tubes made from the composite article previously described can have adiameter elongation of less than 35% of the original diameter whensubjected to 125 psi internal pressure at 25° C.

The composite articles can be used to make gaskets, O-rings, bellows andpump diaphragms.

The present invention can be better understood from the followingexamples and comparisons. It should be understood that the scope of thisinvention is not limited by these specific examples.

All patents and references cited, herein in this application, areexpressly incorporated into this application by reference thereto.

EXAMPLE 1

ePTFE membrane (GORE-TEX® membrane, produced by W.L. Gore & Associates,Inc., Newark, Del.), was coated with liquid silicone (See FIG. 2A) usinga gravure roll (A) and a rubber roll (B) held at a pressure of 90 psi.The membrane was 1.5 mils thick, 30″ wide, and was obtained as acontinuous roll. The membrane had a density of 0.44 g/cc and a mean poresize was 0.25 microns. The liquid silicone was prepared as a mixture of725 gm of α, ω vinyldimethyl endcapped dimethyl siloxane polymer (1,000cps), 9.6 gm of tetrakis (dimethylsiloxy) silane crosslinker, 1.3 gm of1-ethynyl-1-cyclohexanol inhibitor, and 1.75 gm of platinum catalyst (1weight % platinum in vinyl endcapped dimethylsilicone oil). Noreinforcing agent was employed.

The membrane was coated at room temperature at a speed of 5 feet perminute and taken up onto a 3.5 inch mandrel until a wall thickness of 90mils was obtained. No top coating of silicone was employed. Next, theuncured composite was slit down the length of the mandrel and thematerial and laid open as a sheet. Preforms, measuring 5″ wide×5″long×0.090″ thick, were die-cut from the sheet and compression molded ina flat plaque mold at 125° C. using 30,000 lbs of load to prepare 0.075″thick sheets of cured composite elastomer.

The flexure endurance of the above silicone/ePTFE composite elastomerwas compared to the flexure endurance of the best available constituentmaterials: silicone rubber and ePTFE sheeting. Commercial siliconerubber (LIM® 6745 liquid silicone elastomer produced by General ElectricSilicones, Waterford, N.Y.) and ePTFE gasket sheeting (GORE-TEX® GRSheet®, produced by W.L. Gore & Associates, Inc., Newark, Del.) werecharacterized using a flexure endurance apparatus, as discussed in theASTM D2176-69 method. The specimens, measuring 0.600″ wide, 0.075″thick, and 6″ long, were secured in the test apparatus, loaded with a 2Kg mass, and flexed at a rate of 66 cycles/minute over a 180 degree arc.

FIG. 3 illustrates the dramatic difference in flexure resistance of thecomposite elastomer, “Sample A”, when compared with the ePTFE sheeting,“Sample B”, and the commercial silicone rubber, “Sample C”. Thecomposite elastomer lasted over 40 million cycles (still in test)compared to 15 million cycles for the ePTFE sheeting and 0.5 millioncycles for the silicone rubber. This surprising synergistic effect ofthe composite being superior to the individual components appears to berelated to the ability of the composite to transfer stress from onepoint to another on a molecular level throughout the continuousmicrostructure of nodes interconnected by fibrils.

EXAMPLE 2

A series of three composites were prepared in sheet form to comparetheir physical properties with the pure silicone elastomer precursors.First, the composite from example 1 (“Sample A”) was prepared. Second,composite elastomer, “Sample D”, was prepared using the method ofExample 1, and utilized RTV 615 (General Electric Silicones, Waterford,N.Y.) as the liquid silicone elastomer. The third composite, “Sample E”,was prepared using the method of Example 1, and utilized RTV 863(General Electric Silicones, Waterford, N.Y.) as the liquid siliconeelastomer. Table 1 summarizes the ASTM D-412 tensile strength,elongation to break, and 25 % modulus with the pure silicone “Sample F”,RTV 615 (“Sample G”), and RTV 863 (“Sample H”). Also included is theASTM D-2240 Shore A hardness and the ASTM D-624 die B tear strengthvalues for all the materials.

For the ASTM D-412 measurement for tensile strength, dumbbell specimenswere first cut from 0.075″ compression molded sheets using ASTM D412method “C”. Samples were then tested in tension using an Instron tensilemachine (Model 5567) operating at a strain rate of 20″/min., all at roomtemperature.

For the ASTM D-624 measurement for tear strength, specimens were firstcut from 0.075″ compression molded sheets using ASTM D-624 die ABE.Samples were then tested using an Instron tensile machine (Model 5567)operating at a strain rate of 20″/min., all at room temperature.

For the ASTM D-2240 measurement for durometer hardness, specimens fromthe ASTM D-412 test were stacked 3 high making a total thickness ofapproximately 0.225″. The stacked samples were then tested and rankedusing the “Type A” scale.

The composites were determined to be 4 times stronger, 40 times stifferin tension, and were 6 times more resistant to tearing than the mosthighly reinforced silicone, RTV 863. These properties were obtainedusing a composite having a volume fraction of 80-83 % silicone.

EXAMPLE 3

A peristaltic pump tube (“Sample I”) (1.125″ OD and 0.75″ ID) wasprepared by coating ePTFE membrane (GORE-TEX® membrane, produced by W.L.Gore & Associates, Inc., Newark, Del.) using the process illustrated inFIG. 2B. The membrane was passed between the gravure roll (A) and thesilicone rubber roll (B) using a pressure of 90 psi and then passedthrough a 3 mil gap (containing RTV 615) between the chrome roll (C) andanother chrome roll (D). The membrane was 1.0 mil thick, 30″ wide, andwas obtained as a continuous roll. The membrane had a density of 0.44g/cc and a mean pore size was 0.25 microns. The liquid silicone was RTV615 (General Electric Silicones, Waterford, N.Y.) having a viscosity of4,000 cps at 25° C. The membrane was coated at a speed of 10 feet perminute and taken up onto a 0.75″ OD mandrel (E) covered with a 2 milthick skived PTFE release liner until a wall thickness of 188 mils wasobtained. The final top coat thickness of 1.5 mils was obtained to givean overall elastomer content of 90 volume %. The ratio of the thicknessof an elastomer layer to the thickness of an impregnated membrane layerwas 1.5. Next, the uncured composite was placed in a convection oven at150° C. for 20 minutes and removed from the mandrel. The pump tubing waspost-baked for 2 hours at 150° C. to bring about a final cure and removevolatiles. The ratio of wall thickness (0.188 inch) to inner diameter(0.75 inch) was 0.25.

Sample I was cross-sectioned and viewed by scanning electron microscopy.FIG. 4 depicts a scanning electron micrograph of a cross-section takenfrom Sample I. The sample was cross-sectioned using a cold razor bladecutting through the composite at an angle orthogonal to the long axis ofthe ePTFE layers. The volume % silicone was calculated by adding thethickness of the silicone top-coat layer (a) to the thickness ofsilicone in the impregnated layer (b) wherein the silicone content wascalculated from the ratio of ePTFE density to PTFE density which wasthen divided by the total thickness of the composite. In the case ofSample I, the silicone top-coat thickness was 1.5 mils, the densityratio (0.44 g/cc/2.2 g/cc) was 0.2 which yielded a silicone content of1.2 mils in the impregnated layer, which when divided by the totalthickness of 3 mils resulted in a volume % of 90.

The composite elastomer tubing, “Sample I”, exhibits much improved hoopstrength and resistance to dilation, or diameter elongation, whensubjected to internal pressure, as seen by the comparison to siliconerubber and thermoplastic elastomer in FIG. 5. Diameter elongation isdefined as twice the absolute change in radius of the tube as measuredby an LVTD probe placed in contact with the side of the tube. The radiuswas measured 60 seconds after inflation of the water-filled tube affixedto a hydraulic unit. The pressure necessary to dilate the tubes above10% strain defines the pressure handling capacity of the tubes. Thus,the composite elastomer “Sample I”, has more than 4 times the pressurehandling capability of the conventional silicone pump tubing, “SampleJ”, and the thermoplastic elastomer (TPE), MARPRENE® tubing, shown as“Sample K”.

EXAMPLE 4

The composite elastomer tubing of example 3 (“Sample I”), was tested ina Watson-Marlow Model 704 peristaltic pump operating at a speed of 200rpm at 25° C. using water as the liquid medium and compared with thecommercial silicone tubing (“Sample J”). As depicted in FIG. 6, thesilicone tubing ruptured at approximately 300 hours and the compositeelastomer failed after approximately 3,400 hours. In addition to theexceptional life, the composite elastomer exhibited stable flow ratebehavior throughout the duration of the test.

EXAMPLE 5

The composite elastomer tubing of example 3 (“Sample I”), was alsocompared to tubing prepared by the process of Safford (U.S. Pat. No.2,710,290) wherein PTFE powder was compounded into the liquid siliconeelastomer. The composite tubes were compared to the pure elastomer tubesmade from RTV 615 (“Sample L”) and RTV 863 (“Sample M”). Two types ofPTFE powder were compounded into RTV 615A: a non-fibrillating PTFEpowder (TEFLON® 6B fluoropolymer available from E.I. duPont de Nemours &Co., Inc., Wilmington, Del.) was used to prepare “Sample N” and afibrillating powder (CD 123 available from ICI) was used to prepare“Sample O”.

A composite (“Sample N”), based on non-fibrillating PTFE, was producedusing the following method. A sigma blade dough mixer was first loadedwith 1.6 lbs. of RTV 615A and then loaded with a total of 0.87 lbs. ofTEFLON® 6 B fluoropolymer in 0.2 lb. increments to yield a 35 weight %concentrate in RTV 615A. Upon reaching high shear and complete wetting,the mixing was continued for 30 minutes at room temperature. Next, thepaste was diluted to 10 weight % total PTFE solids using 7.1 lbs. of RTV615A. The resultant mixture was well mixed, but the PTFE was notnoticeably fibrillated in the RTV 615A. Finally, 400 gm of RTV 615A wasmixed with 40 gm of RTV 615B using a static mixer. The mixture waspumped into a cylindrical metal mold fitted with a 0.75″ diametermandrel and cured at room temperature overnight to produce a 24″ lengthof elastomer tubing of 1.125″ OD and 0.75″ ID. The tubing was removedfrom the mandrel and post-baked in a convection oven for 2 hours at 150°C.

A composite (“Sample O”), based on fibrillating PTFE, was produced usingthe following method. A sigma blade dough mixer was first loaded with1.6 lbs. of RTV 615A and then loaded with a total of 0.87 lbs. of CD123in 0.2 lb. increments to yield a 35 weight % concentrate in RTV 615A.Upon reaching high shear and complete wetting, the mixing was continuedfor 30 minutes at room temperature. Next, the paste was diluted to 10weight % total PTFE solids using 7.1 lbs. of RTV 615A. The resultantmixture was more viscous than the prior resin due to the formation oflong fibrous PTFE material in the RTV 615A. Finally, 400 gm of RTV 615Awas mixed with 40 gm of RTV 615B using a static mixer. The mixture waspumped into a cylindrical metal mold fitted with a 0.75″ diametermandrel and cured at room temperature overnight to produce a 24″ lengthof elastomertubing of 1.125″ OD and 0.75″ ID. The tubing was removedfrom the mandrel and post-baked in a convection oven for 2 hours at 150°C.

All three composite elastomers, Samples “I”, “N”, & “O”, were testedusing a Watson-Marlow Model 704 peristaltic pump operating at a speed of200 rpm at 25° C. using water as the liquid medium. Table 2 summarizesthe pump tubing life of the 3 compositions and compares it to the neatsilicone elastomer tubing. The composite elastomer exhibited 10,000times longer life compared to the TEFLON® 6 B fluoropolymer filledsilicone and 320 times longer life compared to the ICI CD123 filledsilicone. Even at identical PTFE loadings, the composite of thisinvention exhibited superior life as a result of the continuousmicrostructure of nodes interconnected by fibrils. Therefore, not onlycomposition, but composite morphology is critical to flexure endurance.

EXAMPLE 6

The composite elastomer tubing of Example 3 (“Sample I”), was alsocompared to tubing prepared by the process of Tomoda (U.S. Pat. No.4,133,927) wherein a microporous PTFE material was laminated onto arubber substrate. In particular, an ePTFE tube (0.750″ ID×0.754″ OD) wasprepared using the process of Gore (U.S. Pat. No. 3,953,566) and waspulled onto a 0.75″ OD metal mandrel and placed into a cylindrical metalmold. The PTFE tubing had a density of 0.6 g/cc. Next, a liquid silicone(RTV 615 GE Silicones, Waterford, N.Y.) was pumped into the mold andcured overnight. The silicone wicked into the ePTFE, thus creating anintegral pump tube. The tube was removed from the mandrel and post-bakedin a convection oven for 2 hours at 150° C.

Both the composite elastomer (“Sample I”), and the tube made by theprocess of Tomoda (“Sample P”), were tested using a Watson-Marlow Model704 peristaltic pump operating at a speed of 200 rpm at 25° C. usingwater as the liquid medium. The composite elastomer, “Sample I”,exhibited 9 times the pump tubing life (2,400 hours vs. 267 hours)compared to the ePTFE laminated silicone elastomer, “Sample P”. Thelaminated structures are thought to lack the molecular reinforcementprovided by the ePTFE throughout the bulk of the composite. The cause offailure was delamination between the ePTFE tubing and the laminatedrubber base as well as cracking throughout the bulk elastomer about thelong axis of the tube.

EXAMPLE 7

A small diameter peristaltic pump tube (“Sample Q”) (0.375″ OD and 0.25″ID) was prepared by coating ePTFE membrane (GORE-TEX® membrane, producedby W.L. Gore & Associates, Inc., Newark, Del.) using the processillustrated in FIG. 2B. The membrane was passed between the gravure roll(A) and the silicone rubber roll (B) using a pressure of 90 psi and thenpassed through a 2 mil gap (containing RTV 615) between the chrome roll(C) and another chrome roll (D). The membrane was 2.0 mils thick, 30″wide, and was obtained as a continuous roll. The membrane had a densityof 0.4 g/cc and a mean pore size was 0.25 microns. The liquid siliconewas RTV 615 (General Electric Silicones, Waterford, N.Y.) having aviscosity of 4,000 cps at 25° C. The membrane was coated at a speed of 2feet per minute and taken up onto a 0.25″ OD mandrel (E) covered with askived PTFE release liner until a wall thickness of 63 mils wasobtained. The final top coat thickness of 1.5 mils was obtained to givean overall elastomer content of 93 volume %. The ratio of the thicknessof an elastomer layer to the thickness of an impregnated membrane layerwas 0.75. Next, the uncured composite was placed in a convection oven at110° C. for 10 minutes and removed from the mandrel. The pump tubing waspost-baked for 2 hours at 150° C. to bring about final cure and removevolatiles.

For comparison purposes, a silicone solvent imbibed ePTFE tube (“SampleR”) was also prepared using the process of Tu (EP 256,748). First, amonolithic, porous PTFE tube was extruded on top of a solid PTFE coreusing the process of Gore (U.S. Pat. No. 3,953,566) to yield an ePTFEtube with initial dimensions of 0.25″ ID and 0.442″ OD. The mandrelaided in maintaining a constant inner diameter and enabled the materialto be compressed after immersion. Next, a silicone bath was preparedusing a mixture of 1 part of RTV 4010 (Dow Corning, Corp., Midland,Mich.) with 1 part of mineral spirits, by volume. The tube was thenimmersed in the silicone bath to fully penetrate the ePTFE structure anddried at 60° C. for 20 minutes in a forced air convection oven to removethe residual solvent. Finally, the tube was pulled through a heated,conical die (275° C.) at a rate of 3 feet per minute to compress thetubing to a 0.375″ OD and fully vulcanize the elastomer. The resultanttube had a silicone content of approximately 63 volume %.

Also for comparison purposes, an extruded, heat-cured silicone rubbertube (“Sample S”) was obtained from Cole-Palmer Instrument Company,Vernon Hills, Ill. The rubber tubing had an outside diameter of 0.375″and an inside diameter of 0.25″.

All three peristaltic pump tubes were tested using a Cole-Palmer® ModelL/S with Easy-Load® pump head (Cole-Palmer Instrument Company, Chicago,Ill.) operating at 400 rpm using water as the test medium. Theelasticity of the various tubes was also quantified using a tensiletesting machine operating in the compressive mode. The tubing wascompressed to the point of complete closure of the inside diameter(0.25″ travel), whereupon the load was removed from the sample by movingthe cross-head back to the original position. This process was repeated5 times whereupon the final stroke was recorded until the loaddiminished to 1% of the fully compressed load. The difference betweenthe final stroke and the initial 0.375″ gap was recorded as the amountof “set” the sample exhibited and was divided by the total diameter toyield the % rebound loss. This technique was used to quantify howelastic-like the tubes behaved. The results of % rebound loss and pumptubing life are summarized in Table 3.

The composite tube (“Sample Q”) lasted 18 times longer than thecommercial silicone rubber tubing. The solvent imbibed tube exhibited aplastic-like feel and had a 34 % loss in rebound upon repeated closurecompared to 2.4 % loss for the composite elastomer tube. In addition,the imbibed tube appeared to sweat water droplets while operating in thepump housing. Although the tube was compressed during vulcanization, itretained some amount of porosity which led to rapid deterioration inperformance.

EXAMPLE 8

A functionalized silica filled composite elastomer tube (“Sample T”)(1.125″ OD and 0.75″ ID) was prepared by gravure coating a fumed silicafilled ePTFE membrane (prepared by the process of Gore in U.S. Pat. No.4,096,227; U.S. Pat. No. 4,187,390) using the process illustrated inFIG. 2B. The fumed silica filled membrane was prepared by co-coagulatinga hydrophobic fumed silica (Degussa Corporation AEROSIL® 200 treatedwith 1,3-divinyltetramethyldisilazane (PCR Incorporated, Gainesville,Fla.)), with PTFE emulsion. The silica filled PTFE was paste extrudedand expanded into a 2.75 mil thick membrane with 81 % pore volume. Themembrane measured 30 inches wide, and was obtained as a continuous roll.The membrane had a density of 0.4 g/cc and a mean pore size of 0.25microns. The liquid silicone was RTV 615 (General Electric Silicones,Waterford, N.Y.) having a viscosity of 4,000 cps at 25° C. The silicafilled membrane was passed between the gravure roll (A) and the siliconerubber roll (B) using a pressure of 90 psi and then passed through a 4mil gap (containing RTV 615) between the chrome roll (C) and anotherchrome roll (D). The membrane was coated at a speed of 2 feet per minuteand taken up onto a 0.75″ OD mandrel (E) covered with a 1.1 g/cc densityexpanded PTFE release liner until a wall thickness of 188 mils wasobtained. The overall elastomer content was 91 volume %. The ratio ofthe thickness of an elastomer layer to the thickness of an impregnatedmembrane layer was 0.36. Next, the uncured composite was placed in aconvection oven at 100° C. for 30 minutes and removed from the mandrel.The pump tubing was post-baked for 2 hours at 150° C. to bring aboutfinal cure and remove volatiles.

The silica filled composite elastomer (“Sample R”) was tested using aWatson-Marlow Model 704 peristaltic pump operating with a back pressureof 60 psi and pumping at a speed of 200 rpm at 25° C. using water as theliquid medium and compared with the commercial silicone tubing (“SampleJ”). The composite lasted for 35 hours in comparison to the commercialsilicone rubber tubing which ruptured upon reaching a back pressure of40 psi.

EXAMPLE 9

A plasma treated ePTFE membrane was used to prepare another compositeelastomertube (“Sample U”) (1.125″ OD and 0.75″ ID). The surface treatedmembrane was impregnated with silicone using the gravure coating processillustrated in FIG. 2B. The plasma treated membrane was prepared bypassing a commercial ePTFE membrane (GORE-TEX® membrane, produced byW.L. Gore & Associates, Inc., Newark, Del.) through a microwave plasmadischarge chamber (Acton Technologies, Inc., Pittston, Pa.) tofunctionalize the ePTFE with hydroxyl groups. Next, the hydrophilicmembrane was treated with 1,3-divinyltetramethyldisilazane (PCR lncorp.,Gainesville, Fla.) by immersion in the liquid at room temperature for 24hours. The membrane was removed from the liquid and dried in an oven for3 hours at 150° C. The treated membrane was coated using the methodillustrated in FIG. 2B. The membrane was passed between the gravure roll(A) and the silicone rubber roll (B) using a pressure of 90 psi and thenpassed through a 1.75 mil gap (containing RTV 615) between the chromeroll (C) and another chrome roll (D). The membrane was 0.6 mils thick,30″ wide, and was obtained as a continuous roll. The membrane had adensity of 0.32 g/cc and a mean pore size was 0.25 microns. The liquidsilicone was SLE 5700 (General Electric Silicones, Waterford, N.Y.)having a viscosity of 50,000 cps at 25° C. The membrane was coated at aspeed of 4 feet per minute and taken up onto a 0.75″ OD mandrel (E)covered with a 2 mil thick skived PTFE release liner until a wallthickness of 188 mils was obtained. The overall elastomer content wasdetermined to be 88 volume %. Next, the uncured composite was placed ina convection oven at 120° C. for 30 minutes and removed from themandrel. The pump tubing was post-baked for 2 hours at 150° C. to bringabout final cure and remove volatiles.

The composite elastomer tubing, “Sample U”, was tested using aWatson-Marlow Model 704 peristaltic pump operating at a speed of 200 rpmat 25° C. with water as the liquid medium and compared with thecommercial silicone tubing (“Sample J”). The composite elastomer tubinglasted longer than 1,920 hours (still testing) compared to 300 hours forthe commercial silicone tubing.

EXAMPLE 10

Another peristaltic pump tube (“Sample V”) (1.125″ OD and 0.75″ ID) wasprepared by a tape wrapping process. As shown in FIG. 2B, the ePTFEmembrane (GORE-TEX® membrane, produced by W.L. Gore & Associates, Inc.,Newark, Del.) was passed between the gravure roll (A) and the siliconerubber roll (B) using a pressure of 90 psi and then passed through a 2.5mil gap (containing SLE 5700) between the chrome roll (C) and anotherchrome roll (D). The membrane was 30″ wide, and was obtained as acontinuous roll. The membrane had a mean pore size of 0.25 microns. Theliquid silicone was SLE 5700 (General Electric Silicones, Waterford,N.Y.) having a viscosity of 50,000 cps at 25° C. The membrane was coatedat a speed of 3 feet per minute and taken up onto a 3.5″ mandrel (E).The uncured composite was then slit into 15 individual tapes each 2inches wide. One tape at a time was taken up onto a rotating 0.75″diameter mandrel using a machine lathe. The wrapping continued until thewall thickness reached 188 mils. Next, the uncured composite was placedin a convection oven at 120° C. for 30 minutes and removed from themandrel. The pump tubing was post-baked for 2 hours at 150° C. to bringabout final cure and remove volatiles.

The composite elastomer tubing, “Sample V”, was tested using aWatson-Marlow Model 704 peristaltic pump operating with a back pressureof 60 psi and pumping at a speed of 200 rpm at 25° C. with water as theliquid medium and compared with the commercial silicone tubing (“SampleJ”).

EXAMPLE 11

Another composite elastomer peristaltic pump tube, “Sample W”, wasprepared using multiple liquid silicone elastomer precursors andexpanded PTFE membrane. As shown in FIG. 2B, the membrane was firstpassed between the gravure roll (A) and the silicone rubber roll (B)using a pressure of 90 psi whereupon RTV 615 (GE Silicones, Waterford,N.Y.) was impregnated into the membrane. Next, the membrane was passedthrough a 10 mil gap (containing RTV 863 (GE Silicones, Waterford,N.Y.)) between the chrome roll (C) and another chrome roll (D). Themembrane was 2.0 mils thick, 30″ wide, and was obtained as a continuousroll. The membrane had a density of 0.4 g/cc and a mean pore size was0.25 microns. The two liquid silicones, RTV 615 and RTV 863, hadviscosities of 4,000 cps and 60,000 cps, respectively, at 25° C. Themembrane was coated at a speed of 2 feet per minute and taken up onto a0.75″ OD mandrel (E) covered with a 2 mil skived PTFE release lineruntil a wall thickness of 188 mils was obtained. The final top coatthickness of 3 mils was obtained to give an overall elastomer content of92 volume %. The ratio of the thickness of an elastomer layer to thethickness of an impregnated membrane layer was 1.5. Next, the uncuredcomposite was placed in a convection oven at 120° C. for 30 minutes andremoved from the mandrel. The pump tubing was post-baked for 2 hours at150° C. to bring about final cure and remove volatiles.

The composite elastomer tubing, “Sample W”, was tested using aWatson-Marlow Model 704 peristaltic pump operating with no back pressurerestriction and pumping at a speed of 200 rpm at 25° C. with water asthe liquid medium and compared with the commercial silicone tubing(“Sample J”). The composite elastomer tubing lasted 2,400 hours incomparison to the commercial silicone tubing which ruptured at 300 hoursof operation.

EXAMPLE 12

A pump diaphragm, “Sample X”, was prepared by compression moldingsilicone impregnated ePTFE membrane. First, an ePTFE membrane wasgravure coated (See FIG. 2B) with liquid silicone (RTV 863) using agravure roll (A) and a rubber roll (B) held at a pressure of 90 psi andthen passed through a 1.5 mil gap (containing RTV 863) between thechrome roll (C) and another chrome roll (D). The membrane was 0.2 milsthick, 30″ wide, and was obtained as a continuous roll. The membrane hada density of 0.3 g/cc and a mean pore size was 0.21 microns. Themembrane was coated at room temperature at a speed of 4 feet per minuteand taken up onto a 3.5 inch mandrel (E) until a wall thickness of 158mils was obtained. A top coating of 1.2 mils of silicone was applied tothe top of the gravure coated membrane. The ratio of the thickness of anelastomer layer to the thickness of an impregnated membrane layer was 6.Next, the uncured composite was slit down the length of the mandrel andthe material and laid open as a sheet. Preforms, measuring 10 inches indiameter and 158 mils thick, were die-cut from the sheet and compressionmolded in an aluminum diaphragm mold at 100° C. using 80,000 lbs of loadto prepare a 3 dimensional composite elastomer diaphragm.

The diaphragm was fitted to a Yamada NDP-25BT pump and operated for 240hours with 40 psi air pressure and 10 psi back pressure to deliver 14gal/min of water. A pump diaphragm, “Sample Y”, was prepared using pureRTV 863. This elastomer was found to be too weak to be bolted into placein the pump housing for subsequent testing.

EXAMPLE 13

A composite elastomer bellows, “Sample Z”, was prepared by blow moldinga silicone impregnated ePTFE membrane in a closed cavity mold. First, anePTFE membrane was gravure coated (See FIG. 2B) with liquid silicone(RTV 863) using a gravure roll (A) and a rubber roll (B) held at apressure of 90 psi and then passed through a 1.75 mil gap (containingRTV 863) between the chrome roll (C) and another chrome roll (D). Themembrane was 0.2 mils thick, 30″ wide, and was obtained as a continuousroll. The membrane had a density of 0.3 g/cc and a mean pore size was0.20 microns.

The liquid silicone was RTV 863 (GE Silicones, Waterford, N.Y.) having aviscosity of 60,000 cps at 25° C. The membrane was coated at a speed of2 feet per minute and taken up onto a 3.5″ OD perforated metal mandrel(E) covered with an inflatable silicone rubber bladder until a wallthickness of 100 mils was obtained. The final top coat thickness of 1.3mils was obtained to give an overall elastomer content of 97 volume %.The ratio of the thickness of an elastomer layer to the thickness of animpregnated membrane layer was 6.5. Next, the mandrel was placed in aclosed cavity mold, as shown in FIG. 7, and inflated using an airpressure of 60 psi. The mold was then placed in a convection oven at175° C. for 1 hour. The sample was then removed from the mold andpost-baked for 2 hours at 175° C.

EXAMPLE 14

A solvent resistant fluoroelastomer based composite was prepared as“Sample AA”. ePTFE was coated (See FIG. 2C) with 3,000 cpsliquidperfluoro polyether elastomer, sold under the trademark, SIFEL®610 (Shin Etsu Chemical Company, Tokyo, Japan) using a gravure roll (A)and a rubber roll (B) held at a pressure of 90 psi. The coated membranewas then passed between a rubber roll (B) and a chrome roll (C) held ata pressure of 90 psi. The membrane was 1.5 mils thick, 30″ wide, and wasobtained as a continuous roll. The membrane had a density of 0.44 g/ccand a mean pore size was 0.25 microns. The membrane was coated at roomtemperature at a speed of 2 feet per minute and taken up onto a 3.5 inchmandrel (E) until a wall thickness of 100 mils and an elastomer contentof 85 volume % was obtained. Next, the uncured composite was slit downthe length of the mandrel and the material and laid open as a sheet.Preforms, measuring 5″ wide×5″ long×0.100″ thick and compression moldedin a flat plaque mold at 125° C. using 30,000 lbs of load to prepare0.075″ thick sheets of cured composite elastomer. The sheets werepost-baked in a convection oven at 150° C. for 2 hours.

Another composite elastomer, “Sample BB”, was prepared by compressionmolding a methyl silicone impregnated ePTFE membrane. First, an ePTFEmembrane was gravure coated (See FIG. 2B) with liquid silicone (RTV 863GE Silicones, Waterford, N.Y.) using a gravure roll (A) and a rubberroll (B) held at a pressure of 90 psi and then passed through a 3.0 milgap (containing RTV 863) between the chrome roll (C) and another chromeroll (D). The membrane was 0.75 mils thick, 30″ wide, and was obtainedas a continuous roll. The membrane had a density of 0.32 g/cc and a meanpore size was 0.21 microns. The membrane was coated at room temperatureat a speed of 2 feet per minute and taken up onto a 3.5 inch mandrel (E)until a wall thickness of 100 mils was obtained. As the gravure coatedmembrane passed through the 3.0 mil gap, a top coating of 1.8 mils ofsilicone was applied to the membrane to yield an overall elastomercontent of 96 volume %. The ratio of the thickness of an elastomer layerto the thickness of an impregnated membrane layer was 2.4. Next, theuncured composite was slit down the length of the mandrel and thematerial and laid open as a sheet. Preforms, measuring 5″ wide×5″long×0.095″ thick and compression molded in a flat plaque mold at 125°C. using 40,000 lbs of load to prepare 0.075″ thick sheets of curedcomposite elastomer. The sheets were post-baked in a convection oven at150° C. for 2 hours.

Table 4 summarizes the effects of various chemicals on the physicalproperties of the perfluoro polyether elastomer and methyl siliconecomposite elastomers (“Sample A” and “Sample BB”, respectively). Themeasured properties include the ASTM 0412 tensile strength, elongationto break, 25% modulus, and Shore A hardness (ASTM D-2240). In general,the perfluoro polyether elastomer composites retain the chemicalresistance of the perfluoro polyether elastomer base but draw from theePTFE structure to dramatically increase their strength and toughness,even after extended immersion in harsh chemicals.

EXAMPLE 15

A perfluoro polyether elastomer composite pump diaphragm, “Sample CC”,was prepared by compression molding silicone impregnated ePTFE membrane.First, an ePTFE membrane was gravure coated (See FIG. 2C) with liquidperfluoro polyether elastomer (SIFEL® 610) using a gravure roll (A) anda rubber roll (B) held at a pressure of 30 psi and further conveyedbetween a rubber roll (B) and a chrome roll (C) held at 30 psi. Themembrane was 0.2 mils thick, 30″ wide, and was obtained as a continuousroll. The membrane had a density of 0.3 g/cc and a mean pore size was0.25 microns. The membrane was coated at room temperature at a speed of5 feet per minute and taken up onto a 3.5 inch mandrel (E) until a wallthickness of 145 mils was obtained. No top coating of elastomer wasemployed. Next, the uncured composite was slit down the length of themandrel and the material and laid open as a sheet. Preforms, measuring10 inches in diameter and 145 mils thick, were die-cut from the sheetand compression molded in an aluminum diaphragm mold at 150° C. using30,000 lbs of load to prepare a 3 dimensional composite elastomerdiaphragm having an elastomer content of 83 volume percent.

EXAMPLE 16

A perfluoro polyether elastomer composite bellows, “Sample DD”, wasprepared by blow molding a perfluoro polyether elastomer impregnatedePTFE membrane in a closed cavity, corrugated, aluminum mold. First, anePTFE membrane was gravure coated (See FIG. 2B) with liquid perfluoropolyether elastomer (SIFEL® X-70-701) using a gravure roll (A) and arubber roll (B) held at a pressure of 90 psi and further conveyedbetween a rubber roll (C) and a chrome roll (D) held at 90 psi. Themembrane was 0.2 mils thick, 30″ wide, and was obtained as a continuousroll. The membrane had a density of 0.3 g/cc and a mean pore size was0.25 microns. The membrane was coated at a speed of 4 feet per minuteand taken up onto a 3.5″ OD porous metal mandrel (E) covered with asilicone rubber bladder until a wall thickness of 100 mils was obtained.The overall elastomer content was 90 volume %. Next, the mandrel wasplaced in a closed cavity mold, as shown in FIG. 7, and inflated usingan air pressure of 60 psi. The mold was then placed in a convection ovenat 175° C. for 1 hour. The sample was then removed from the mold andpost-baked for 2 hours at 175° C.

EXAMPLE 17

A solvent resistant peristaltic pump tube (“Sample EE”) (1.125″ OD and0.75″ ID) was prepared using the process illustrated in FIG. 2B. AnePTFE membrane (GORE-TEX® membrane, produced by W.L. Gore & Associates,Inc., Newark, Del.) was passed between a gravure roll (A) and a siliconerubber roll (B) using a pressure of 90 psi and then passed through a 2.5mil gap (containing SIFEL® X-70-709)between a chrome roll (C) andanother chrome roll (D). The membrane was 0.75 mils thick, 30″ wide, andwas obtained as a continuous roll. The membrane had a density of 0.32g/cc and a mean pore size was 0.25 microns. The liquid perfluoropolyether elastomer was sold as SIFEL® X-70-709 (Shin Etsu ChemicalCompany, Tokyo, Japan) having a viscosity of 50,000 cps at 25° C. Themembrane was coated at a speed of 2 feet per minute and taken up onto a0.75″ mandrel (E). The uncured composite was then compression molded ina closed cavity clam shell mold to yield a tube with a 188 mil wallthickness and elastomer content of 83 volume percent. The mold wasplaced in a convection oven at 120 OC for 60 minutes. The pump tubingwas removed from the mold and post-baked for 2 hours at 150° C. toaffect final cure and remove volatiles.

The composite elastomer tubing, “Sample EE”, was tested using aWatson-Marlow Model 704 peristaltic pump operating with no back pressureand pumping at a speed of 200 rpm at 25° C. with water as the liquidmedium. VITON® pump tubing is not commercially available in largediameter sizes.

EXAMPLE 18

A series of conformable films were prepared by combining varioussilicone and perfluoro polyether elastomers with expanded PTFE membranesas summarized in Table 5. All films were prepared, as illustrated inFIG. 2B, by passing an ePTFE membrane (GORE-TEX®membrane, produced byW.L. Gore & Associates, Inc., Newark, Del.) between a gravure roll (A)and a silicone rubber roll (B) using an applied pressure and then,optionally, passing the imbibed membrane through a gap (containingliquid elastomer) between a chrome roll (C) and another chrome roll (D).Next, the coated membranes were passed through a forced air convectionoven (180° C.) to vulcanize the elastomer composites.

The membrane, liquid elastomer, and composite elastomer properties aresummarized in Table 5. The composite elastomer films were alsocharacterized with respect to their oxygen permeability using ASTMD1434-92, procedure V. Samples were loaded in a gas permeability cell(Custom Scientific Model CS-135) to provide a cross-sectional area of 66cm². Oxygen was applied (5 psig) to one side of the mounted films at 25°C. in order to measure the oxygen permeabilities, as summarized in Table5.

The composite elastomer samples were repeatedly drawn into a multiplecavity mold, as depicted in FIG. 8. Due to the toughness, tearresistance, and flexure fatigue properties of these composites, all ofthe samples were capable of withstanding more than 25 repeated cycles ofdrawing and releasing. In contrast, the commercially available releasefilms including skived PTFE (2 mil) and ETFE (TEFZEL®) were permanentlydeformed after single use and developed small perforations in the filmsupon drawing into the mold.

EXAMPLE 19

A composite elastomer gasket, “Sample KK”, was formulated and comparedto a commercial silicone rubber gasket, “Sample LL”, and an expandedPTFE gasket, “Sample MM” (GORE-TEX GR® sheet gasketing available from W.L. Gore & Associates, Inc., Newark, Del.). The composite elastomer,“Sample KK”, was prepared by gravure coating and subsequent compressionmolding. An ePTFE membrane (GORE-TEX® membrane, produced by W.L. Gore &Associates Inc., Newark, Del.), was coated with liquid silicone, RTV 615(GE Silicones, Waterford, N.Y.), by passing the membrane between agravure roll (A) and a rubber roll (B) held at a pressure of 90 psi, asillustrated in FIG. 2C. The membrane was then passed between rubber roll(B) and chrome roll (C) at a pressure of 90 psi to improve penetrationof the liquid silicone into the ePTFE structure. The membrane was 1.5mils thick, 30″ wide, and was obtained as a continuous roll. Themembrane had a density of 0.44 g/cc and a mean pore size was 0.25microns. The membrane was coated at room temperature at a speed of 5feet per minute and taken up onto an 8 inch mandrel until a wallthickness of 156 mils was obtained. Next, the uncured composite was slitdown the length of the mandrel and the material and laid open as asheet. A preform, measuring 5.25″ wide×5.25″ long×0.156″ thick, wasdie-cut from the sheet and compression molded in a flat plaque mold at125° C. using 30,000 lbs of load to prepare a 0.125″ thick sheet ofcured composite elastomer. A ring shaped specimen was die cut from thesheet to yield a gasket with a 3 inch OD and a 2 inch ID.

The water sealability of the three gaskets was tested according to theASTM F-37 method wherein the samples were secured in a 3″×0.150″ ANSIring, mounted in a hydraulic press, and challenged with 30, 60, and 100psi of pressurized water at room temperature. Compression stress wasapplied to the fixture until no leaking was detected around the gasket.The stress values necessary to seal the particular gaskets were recordedand summarized in Table 6.

The elastomer composite required ½ the stress to seal compared to theePTFE gasket and comparable stress to seal relative to the siliconerubber gasket. Thus, the composite elastomer with its low stress to sealcoupled with its 6 times greater tear strength and 4 times greatertensile strength (see Table 1), provides for simultaneous conformabilityand toughness.

Without intending to limit the scope of the present invention, theforegoing examples illustrate how the present invention may be made andused.

While particular embodiments of the present invention have beenillustrated and described herein, the present invention should not belimited to such illustrations and descriptions. It should be apparentthat changes and modifications may be incorporated and embodied as partof the present invention within the scope of the following claims.

TABLE 1 SILICONE REINFORCE- VOLUME % TENSILE ELONGATION MODULUS HARDNESSTEAR SAMPLE GRADE MENT SOURCE SILICONE STRENGTH (PSI) (%) (@ 25%, PSI)(SHORE A) (PPI) A PDMS Unreinforced 83 2,700 195 1,430 86 606 D GE RTV615 Resin 80 3,700 190 2,470 88 770 E GE RTV 863 Fumed Silica 80 3,500250 2,500 85 850 F GE PDMS Unreinforced 100 200 50 — 40 0 G GE RTV 615Resin 100 1,050 125 86 54 0 H GE RTV 863 Fumed Silica 100 900 320 62 48137

TABLE 2 SILICON PTFE VOLUME % TUBING SAMPLES GRADE GRADE SILICONE LIFE(HRS.) I GE RTV 615 Gore ePTFE 94.0 2,400 N GE RTV 615 DuPont 6B 94.50.25 O GE RTV 615 ICI CD123 94.5 7.50 L GE RTV 615 None 100 7.0 M GE RTV863 None 100 70

TABLE 3 SAM- SILICONE VOLUME % % REBOUND TUBING PLES GRADE SILICONE LOSSLIFE (HRS.) Q GE RTV 615 93 2.4 1,250 R Dow Corning 60 34 136 RTV 4010 SHeat Cured 100 2.4 68 Rubber

TABLE 4 CHANGE IN HARDNESS % VOLUME CHANGE % TENSILE CHANGE % ELONGATIONCHANGE (POINTS) SOLVENT SAMPLE AA SAMPLE BB SAMPLE AA SAMPLE BB SAMPLEAA SAMPLE BB SAMPLE AA SAMPLE BB Hexane 3 159 −3 5 −2 −41 −2 −20 Toluene3 114 −3 −2 0 −55 −2 −17 Tetra- 7 129 −1 0 −5 −61 −1 −22 hydrofuranSilicone Oil 0 11 −5 −1 7 −21 −1 −3 Perfluoro Ether 93 3 −10 9 −16 4 −90 ASTM Fuel C 4 145 −1 −18 −1 −80 −1 −28 Gasoline 4 155 −9 −14 10 −75 0−22 Gear Oil 0 6 −5 −8 0 −32 1 0 Methanol 1 0 −6 −2 10 −14 0 −1 MethyEthyl 5 61 −8 −11 −10 −58 0 −12 Ketone Acetone 4 23 −1 −3 −9 −25 −2 0Hydrochloric 0 1 −3 −19 5 −71 0 −3 Acid Sulfuric Acid −4 Degraded −11Degraded 31 Degraded −7 Degraded Nitric Acid 2 0 −1 −29 1 −63 1 0 Sodium0 2 2 −21 −7 −71 0 1 Hydroxide

TABLE 5 OXYGEN VOLUME % ELASTOMER PERMEABILITY COMPOSITE MEMBRANE SAMPLEELASTOMER GRADE (cm²/sec/atm) THICKNESS (mils) THICKNESS (mils) FF* 80GE RTV 863 1.14 × 10⁻⁴ 3.0 1.0 GG 94 DC Q36679 1.09 × 10⁻⁶ 2.5 0.75 HH80 GE RTV 615 4.61 × 10⁻⁶ 3.5 3.5 II 91 GE RTV 615 3.54 × 10⁻⁶ 1.1 0.5JJ 97 SIFEL ® 610 7.49 × 10⁻⁷ 3.0 0.5 *Material was porous through ½ thethickness direction and resulted in leak through, and thus, above thedetection limit of the cell.

TABLE 6 VOLUME STRESS STRESS STRESS SAM- OF ELAS- TO SEAL TO SEAL TOSEAL PLE MATERIAL TOMER AT 30 PSI AT 60 PSI AT 100 PSI KK Elastomer 80110 120 240 Composite LL Silicone 100 110 140 170 Rubber MM ePTFE 0 200300 450

We claim:
 1. A composite tube comprising: a plurality of expanded PTFElayers having a node and fibril structure, wherein the expanded PTFElayers are impregnated with at least one elastomer, and layers ofelastomer, wherein the impregnated layers are adhered together by thelayers of elastomer and wherein the ratio of the thickness of anelastomer layer to the thickness of an impregnated expanded PTFE layeris 6.5:1 or less.
 2. The composite tube of claim 1, wherein the tube hasa diameter of elongation of less than 35% of the original diameter whensubjected to 125 psi internal pressure at 25° C.
 3. The tube of claim 2wherein the ratio of wall thickness to inside diameter is less than0.25.
 4. The composite of claim 1, wherein said at least one elastomerand the layers of elastomer each independently comprise a silicone, aurethane, a nitrile rubber, a styrene-butadiene rubber, a chloroprene, aphosphazene, a fluoroelastomer, a perfluoroelastomer, a perfluoropolyether elastomer or combinations thereof.
 5. The composite tube ofclaim 1, wherein said at least one elastomer and the layers of elastomereach independently comprise an organosilicone.
 6. The composite tube ofclaim 1, wherein said at least one elastomer and the layers of elastomereach independently comprise a methyl silicone, a phenyl silicone, afluorosilicone, or combinations thereof.
 7. The composite tube of claim1, wherein said at least one elastomer and the layers of elastomer eachindependently comprise a fluoroelastomer, a perfluoroelastomer, aperluoro polyether elastomer, or combinations thereof.
 8. The compositetube of claim 1, wherein the volume ratio of elastomer to expanded PTFEis at least 4:1.
 9. The composite tube of claim 1, wherein said expandedPTFE includes at least one filler.
 10. The composite tube of claim 1,wherein said at least one filler is fumed silica, colloidal silica,carbon black, or combinations thereof.
 11. The composite tube of claim1, wherein said expanded PTFE layers include a plasma treatment.
 12. Thecomposite tube of claim 1, wherein said expanded PTFE layers have asilane coupling agent treatment thereon.
 13. The composite tube of claim1, wherein the tube can be continuously operated in a peristaltic pumpfor at least 100 hours at 200 rpm at room temperature at a pressure ofat least 30 psi.
 14. A composite tube comprising at least 31 layers ofexpanded PTFE having a node and fibril structure, wherein the expandedPTFE layers are impregnated with at least one organosilicone elastomer,and layers of elastomer, wherein said impregnated layers are adheredtogether by the layers of organosilicone elastomer, and wherein theratio of the thickness of an organosilicone elastomer layer to thethickness of an impregnated expanded PTFE layer is 6.5:1 or less. 15.The composite tube of claim 14, wherein said expanded PTFE furthercomprises fumed silica filler.
 16. The tube of claim 14, wherein thetube is non-contaminating, steam sterilizable and can be continuouslyoperated in a peristaltic pump for at least 100 hours at 200 rpm at roomtemperature at a pressure of at least 30 psi.
 17. A composite tubecomprising: a plurality of expanded PTFE membrane layers having amicrostructure of nodes interconnected by fibrils, the microstructure ofthe expanded PTFE layers impregnated with at least one elastomer, andlayers of elastomer, wherein the impregnated expanded PTFE layers areadhered together by the layers of elastomer, and wherein the ratio ofthickness of a layer of elastomer to the thickness of an impregnatedexpanded PTFE layer is 6.5:1.
 18. A composite tube comprising at least31 layers of expanded PTFE membrane having a microstructure of nodesinterconnected by fibrils, wherein the layers are impregnated with atleast one organosilicone elastomer, and layers of organosiliconeelastomer, wherein the impregnated expanded PTFE membrane layers areadhered together by the layers of organosilicone elastomer, wherein theratio of thickness of a layer of organosilicone elastomer to thethickness of an impregnated expanded PTFE membrane layer is 6.5:1 orless.